1. Field of the Invention
The present invention relates to a polycrystalline silicon ingot casting mold and a method for producing the same, silicon nitride powder for mold release material for a polycrystalline silicon ingot casting mold and a slurry containing the same.
2. Description of the Related Art
As one type of semiconductor substrate for producing a solar cell, polycrystalline silicon is widely used and its production amount rapidly increases every year. Polycrystalline silicon is generally formed either by filling silicon melt, which is melt by heating at high temperature, in a mold formed of a quartz crucible, a dividable graphite crucible, or a quartz crucible installed inside a graphite crucible, the inner surface of which is coated with a mold release material by spray or by using a brush or a spatula, followed by solidification, or by melting first a silicon material added in the mold followed by re-solidification.
The release layer prevents incorporation of impurities to a silicon ingot and adhesion of heat-melted silicon melt onto an inner surface of a crucible as a mold, and thus plays an important role of releasing a solidified silicon ingot from a mold. As for the mold release material, from the viewpoint of having generally high melting point and little contamination for silicon ingot, high purity powder such as silicon nitride, silicon carbide, and silicon oxide, or their mixture powder is used. For increasing productivity of silicon ingot, many researches and studies have been made regarding a method of forming a mold release material layer on a mold surface, a mold obtained by such treatment, or a method for producing a silicon ingot by using such mold.
For example, Patent Literature 1 discloses a release layer of a mold for casting a silicon ingot which is formed of silicon nitride powder obtained by thermal decomposition of silicon diimide and fine silica powder as a mold release material and includes a first layer located on an inner side (i.e., mold side) of the release layer and having which has high ratio of silicon nitride containing non-crystalline silica layer on surface of silicon nitride particles, and a second layer located on an outer side (i.e., silicon ingot side) of the release layer and containing fine silica powder mixed therewith so that powder particles are strongly bound to each other. It is also disclosed that, due to the presence of the release layer, damages occurring on a solidified silicon ingot and a mold can be reduced during release. In addition, it is disclosed in Patent Literature 2 that, by using a release layer based on a mixture in which coarse melt silica sands are coated with silicon nitride powder, reaction between molten silicon and silica is inhibited so that the performance of a battery for solar power generation can be enhanced.
[Patent Document 1] JP 2005-95924 A
[Patent Document 2] JP 2001-198648 A